In recent years, dies are increasingly used for the high-temperature forging of hardly workable metal materials such as titanium alloy and Ni-based alloy. In a forging die device conventionally provided in a forging press apparatus for high-temperature forging, for example, a plurality of heater insert holes are formed in each of an upper die and a lower die. The upper die and the lower die are heated by rod-shaped sheath heaters (called also “cartridge heaters”) inserted respectively to the heater insert holes. This type of forging die devices is shown, for example, in Patent Documents 1 and 2.